复合函数求导外导乘以内导y'=x^(2/3)+2/3(x-1)x^(-1/3) =x^(2/3)+2/3*x^(2/3)-2/3*x^(-1/3) =5/3*x^(2/3)-2/3x^(-1/3) y''=5/9*x^(-1/3)+2/9x^(-4/3)
